The function of a typical motor speed controller is to regulate the speed of a connected motor to a desired value and to limit or regulate motor current to a safe level. To accomplish these functions thyristor type controllers must include not only regulator circuitry but also thyristor firing circuitry and line synchronization circuitry which fixes the time axis relationship between the thyristor firing pulses and the AC line voltages.
One type of control system is a regenerative control system which is capable of controlling power flow from an AC line to the motor (motoring action) or power flow from the motor back into the AC line (generating action). In regenerative control circuits a thyristor bridge rectifies the AC line voltages and through a conversion process known in the art as phase control applies a fraction of the maximum available rectified line voltage to the motor causing current to flow through the motor which subsequently causes it to rotate. The fraction of available voltage applied to the motor is determined by the magnitude of the phase command. As the motor rotates, a speed feedback output is produced which is proportional to the motor speed.
Regulators, which are generally high gain closed loop systems, drive a phase command to whatever polarity and magnitude is required to force the feedback function to agree with the corresponding command function. The typical loop architecture causes the speed loop to be satisfied except when the current which is required to do so exceeds the current limit command. In any case, the net result is that the regulator section outputs a phase command which controls the fractional part of AC line voltage that the thyristors apply to the motor thereby determining the current which flows through the motor.
In order for the phase command to exercise control as described, each firing pulse generator is synchronized to the appropriate AC line by an "enable" timing interval which is typically of 180 electrical degrees duration. During this interval, timing of the firing pulse for a connected thyristor occurs under control of a phase command signal.
In the prior art methods, timing intervals start and end at the zero crossings (zero voltage points) of a given line to line AC voltage. This method is suitable for controlling thyristor bridges which control the current to resistive or inductive loads which do not contain an active voltage component such as the counter EMF of a rotating DC motor. When prior art timing methods are utilized with thyristor bridges controlling DC motors, however, the counter EMF of the rotating motor causes the magnitude of the phase command required to produce zero current to the motor to increase proportional to the counter EMF of the motor. This occurs because, in the absence of thyristor conduction, the polarity of voltage across a given thyristor, and thus the period in which it may be fired, is determined not by just a line voltage, but by the sum of a line voltage and the counter EMF of the motor.
This load-voltage-induced offset in the phase command is undesirable for several reasons, particularly if the thyristor bridge is bidirectional and capable of producing either polarity of current to the motor. Such bridges are actually two independent back-to-back bridges and logic must be provided in prior art controllers to select the correct bridge to satisfy the regulator loop which is providing the phase command. If it were not for the load-voltage-induced offset in the phase command, the polarity of the phase command could be used to directly control the polarity of the motor current and bridge selection logic would not be necessary. Other disadvantages caused by the zero offset of prior art controllers include inability of the controller to safely start a rotating motor, snap-on and dead band effects, and potentially damaging current surges that can occur if the phase command is forced to zero as a means of protective shutdown of the controller.
Thus, there is a need in the art for an improved functional block within a thyristor control system for DC motors or for control of the DC bus voltage of AC variable frequency motor controls of the voltage-source type. Specifically, there is a need in the art for circuitry which accurately synchronizes thyristor firing pulses to AC lines when the connected load includes an active voltage component.